The invention relates to a color display tube comprising, in an evacuated envelope, means for generating a number of electron beams, a display screen having a large number of areas luminescing in different colors when struck by the beams, and color selection means for associating each electron beam with luminescent regions of one color. The color selection means comprises a metal plate which is provided with rows of apertures and which forms a first set of lens electrodes to which, between the rows, spherical or rod-shaped insulating carriers are fixed by an adhesive, to which carriers, elongate conductors are secured which constitute a second set of lens electrodes. Between the sets, a potential difference is applied.
Such a color display tube is disclosed in Netherlands Patent Application No. 7600420 laid open to public inspection. By applying a potential difference between the elongate conductors and the metal plate, the apertures form electrostatic quadrupole lenses each having an electric field which is perpendicular or substantially perpendicular to the electron beams passing through the apertures. The lenses focus the electron beams in one direction and defocus them in the direction extending at right angles thereto. The color selection occurs analogous to color display tubes without post-focusing. As a result of post-focusing, however, the apertures may be much larger than in tubes without post-focusing so that a greater number of electrons impinges upon the display screen. As a result, a much brighter or sharper picture can be obtained because sufficient brightness is possible with a smaller electron beam current. In fact, with a smaller beam current, a small spot is possible substantially without a haze surrounding the spot. Such a color display tube is described elaborately in U.S. Pat. No. 4,059,781 which is considered incorporated by reference.
Netherlands Patent Application No. 7600420 laid open to public inspection discloses a color display tube in which the metal plate, at least between the rows of apertures, is provided with a layer of adhesive material, for example, consisting of the polyamide of 4-4' diaminodiphenyl ether and 1-2-4-5 benzenetetracarboxylic acid dianhydride, of methylmethacrylate resin or of sealing glasses or polymers. On their sides facing the plate, the elongate conductors are also covered with such a layer of adhesive material. The elongate conductors are kept at a defined distance from the plate by grains of an electrically insulating material. The grains are sunk partly in the layer of adhesive material present on the plate and partly in the layer of adhesive material present on the elongate conductors. Such a construction has the disadvantage that, in the case of color selection means of large dimensions (having a diagonal exceeding 35 cm), it is difficult to position the elongate conductors accurately between the rows of apertures in the plate without color defects arising in the picture.
U.S. Pat. No. 2,916,649 discloses a construction for the accurate positioning of a number of electrodes relative to each other in travelling wave guides. The electrodes comprise pits, holes or channels obtained by deep drawing in which spacer elements of insulation material are placed. In color selection means for display tubes as hereinbefore described, such a construction cannot be used. 300,000 holes or pits would be necessary in the color selection means. Moreover, positioning two electrodes relative to each other by means of holes or pits is not unambiguous. The spacer elements will always engage in the holes or pits in a different manner. Providing channels by means of deep drawing is also not desired since this makes the color selection means much less rigid in one direction. Moreover, sharp edges are formed in such channels which might give rise to flash-over between the elongate conductors and the plate if they were used in the color selection means. The distance between the conductors and the apertured metal plate in such color selection means is, in fact, 100 .mu.m and the potential difference is approximately 2000 Volts.
Netherlands Patent Application No. 7600422 laid open to public inspection discloses a color display tube in which elongate conductors are positioned between rows of apertures in a metal plate and are each kept at a defined place by at least one insulating member. The member consists of a core which determines the distance between the elongate conductors and the plate and a jacket which is adhered directly to the elongate conductors and the plate. The core consists of a material having a melting point higher than the jacket material. The core consists, for example, of a glass fiber and is surrounded by a jacket of glass having a melting point lower than the glass fiber. This construction also has the disadvantage that, in shadow masks of large dimensions, it is difficult to position the elongate conductors accurately between the rows of apertures in the metal plate. A very accurate positioning is necessary in order to obtain a pure picture.